If you have only one day to dive Cozumel and you have dove within the last year and are comfortable in diving perhaps you should try one of the top end boutique / valet ops. The dive your tank op’s allow 70+ minute dives (AL80’S), if you have the air consumption rate to support it.
If I were to front end my trip from the mainland with a dive on Coz I would travel over the afternoon before and get set up with the operator and then get a cheap (decent) room like Suites Colonial ($40). This way you can have a nice local dinner maybe a few drinks and relax and get a good night’s sleep prior to diving without scrambling over in the AM.
I am partial to Tres Pelicanos (180 dives in the last 14 months) and think they are the best operator on the island but I have also heard very nice things about Steve and Kami with Liquid Blue Divers, Dive with Alison, Living underwater and the list goes on. I would avoid the ‘vapor’ operators that sell dives from the mainland and then farm the dive out to the lowest cost boat available.
I would try to make a reservation soon, last week in what was supposed to be ‘off season’ the boats filled up and multiple ops were turning away divers, the normal ‘extra’ rental boats were booked and unavailable. I think December will be a very busy season on the island.